In the construction of pneumatic tires, it is believed to be virtually impossible to economically manufacture an absolutely uniform tire because of the many variables involved in a tire's construction. Consequently, pneumatic tires, as manufactured, almost inevitably possess a certain degree of non-uniformity. The effects of non-uniformity are best explained by noting that several types of forces are simultaneously exerted by a tire during its rotation under load against a surface. For example, lateral forces, which are of particular importance in the present application, are exerted in the axial direction of the tire or in a direction parallel to its plane of rotation. In a non-uniform tire, the lateral forces exerted by the tire will vary or change during its rotation. In other words, the magnitude and/or direction of the lateral force exerted by the tire will depend on which increment of its tread is contacting the surface.
The variation in lateral force during rotation of a tire, or "lateral force variation", is usually caused by differences in the stiffness and/or geometry of the tire about its circumference, or tread. If these differences are slight, the lateral force variation will be insignificant and its effects unnoticeable when the tire is installed on a vehicle. However, when such differences reach a certain level, the lateral force variation may be significant enough to cause rough riding conditions and/or difficult handling situations.
Consequently, methods have been developed to correct for excessive lateral force variation in a pneumatic tire by removing material from the shoulders of the tire tread. Most conventional lateral force variation correction methods include the steps of indexing the tire tread into a series of circumferential increments and obtaining a series of lateral force measurements representative of the lateral force exerted by the tire as these increments contact a surface. This data is then interpreted and material is removed from the tire tread in a pattern related to this interpretation.
Lateral force variation correction methods are commonly performed with a tire-uniformity machine which includes an assembly for rotating a test tire against the surface of a freely rotating loading drum. This arrangement results in the loading drum being moved in a manner dependent on the forces exerted by the rotating tire whereby lateral forces may be measured by appropriately placed measuring devices. In a sophisticated tire-uniformity machine, the lateral force measurements are interpreted by a computer and material is removed from the tire tread by grinders controlled by the computer.
The interpretation step of a lateral force correction method usually entails the generation of waveform data. This waveform data may be viewed as an array including a series of increment identifiers (which identify the increment contacting the surface) and a corresponding series of lateral force measurements (which represent the total lateral force exerted by the tire when the identified increment contacts the surface). For analytical purposes, it is often helpful to plot the lateral force measurements against the increment identifiers to generate a "lateral force variation waveform."
The waveform data is used, in some manner, by most lateral force correction methods. However, the interpretation of this data, and the related removal of material, differs from method to method. For example, in U.S. Pat. No. 4,047,338 to Gormish (assigned to the assignee of the present application), the interpretation of the lateral force variation waveform includes approximating it to a harmonic (or sine) curve and removing material from alternate shoulders of the tire treads in 180.degree. lengths according to the shape of this harmonic curve. However, methods which approximate the variation waveform to a harmonic curve sometimes disguise, and thus do not efficiently correct, problems areas of certain waveforms. More particularly, in contrast to a harmonic curve which always has one upper apex and one lower apex, a lateral force variation waveform will often include multiple apexes and other non-harmonic characteristics.
Another procedure for interpreting the waveform and relating this interpretation to material removal is set forth in U.S. Pat. No. 3,946,527 to Beer. In the lateral force correction method disclosed in this patent, the maximum lateral force and the maximum variation from this maximum lateral force are determined by the waveform's deviation from a preset variation range (which is centered about the mean of the point representing the maximum lateral force and the point representing the greatest variation from this force). Material is then removed from the locations on the tire tread corresponding to the maximum force measurement and the maximum variation from this measurement. While the Beer correction method may eliminate some of the inaccuracies associated with harmonic approximation, it does not efficiently accommodate multiple apexes.
For these reasons, a need remains for a method of correcting lateral force variation in a pneumatic tire which efficiently accommodates lateral force variation patterns corresponding to waveforms having multiple apexes and/or other non-harmonic characteristics.